Compliant foil fluid film thrust bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a two sided thrust disk rotating element, non-rotating compliant fluid foil members that axially enclose the rotating element, non-rotating compliant spring foil members that axially enclose the fluid foil members and a non-rotating thrust plate element and a non-rotating housing element that axially enclose and provide attachments for the foil members. The space between the rotating element and the thrust plate element on one side of the bearing and the space between the rotating element and the thrust surface of the housing element on the other side of the bearing are filled with fluid (usually air) which envelops the foils.
The rotary motion of the rotating element applies viscous drag forces to the fluid and induces circumferential flow of the fluid between the smooth surface of the rotating element and the fluid foil. The space between the rotating element and the fluid foil is subdivided into a plurality of fluid-dynamic wedge channels. These wedge channels have typically been formed by resistance welding compliant, convex curved foil pads to an underlying support foil. The leading ramps of the foil pads relative to the fluid's circumferential flow and the smooth surface of the rotating element form the two primary surfaces of the converging wedge channels. The trailing ramps and the smooth surface of the rotating element form the primary surfaces of the diverging wedge channels. The fluid flowing circumferentially along a converging wedge channel experiences steadily decreasing flow area, increasing circumferential flow velocity and increasing static fluid pressure. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases causing the fluid pressure rise along the channel to increase. If the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Conventional, compliant foil fluid film thrust bearings have fluid dynamic wedge channel ramps that converge or diverge circumferentially with no radial component to the ramp slopes. The converging wedge channel ramps have no side wall or other constraints to prevent fluid flow out of the channels at their inner and outer edges. At the trailing edge of the converging wedge channel, the high fluid pressure and lack of radial flow constraints induces radial flow leakage out of the channel, which in turn, results in a reduction in fluid pressure, a loss in bearing load capacity, and an increase in bearing drag. The radial flow leakage requires make-up flow at the beginning of the converging wedge channel.
Conventional, compliant foil fluid film thrust bearings have primary fluid flow patterns in the converging wedge channels that are single path recirculating loops. The fluid in the converging wedge channels adjacent to the rotating disk travels circumferentially in the same direction as the disk's motion (up the ramp) owing to viscous drag. The fluid in the converging wedge channels adjacent to the non-rotating fluid foil travels circumferentially in the direction opposite to the disk's motion (down the ramp) owing to the circumferential pressure gradient along the channel. Much of the fluid that travels up the ramp near the disk while gaining static pressure turns back before reaching the end of the wedge channel and travels down the ramp near the fluid foil while losing pressure. Almost all of this fluid turns again before reaching the beginning of the wedge channels and travels up the ramp while again gaining pressure. The fluid traveling the single path recirculating loop flow patterns travels essentially the same path each loop and experiences the same pressure increases and pressure decreases each loop with no net pressure gain from one loop to the next. These bearings generate less fluid dynamic pressure and have less load capacity than bearings that utilize multi-path vortex flow patterns where the flow traveling each regenerative loop travels a different path and where there is a net increase in fluid pressure each loop
Conventional, compliant foil fluid film thrust bearings operate with extremely small running clearances and moderate as opposed to low drag and power consumption. The clearances between the non-rotating fluid foil's converging channel ramp trailing ends and the rotating thrust disk are typically less than 100 micro-inches when the bearing is heavily loaded at operating conditions. The bearing's drag coefficient is typically more than 0.005 at operating speed as defined by the ratio of the fluid dynamic drag induced shear forces applied to the disk by the bearing divided by the thrust load carried by the bearing.
Compliant foil fluid film thrust bearings tend to rely on backing springs to preload the fluid foils against the relatively moveable rotating element (thrust disk) so as to control foil position/nesting and to establish foil dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the disk speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high lift-off/touch-down speeds result in significant bearing wear each time the disk is started or stopped.
Many conventional, compliant foil fluid film thrust bearings have large sway spaces and loose compliance, i.e. they do not tightly restrict the axial or overturning motion of the bearing thrust disk, owing to poor control of spring deflection tolerances inherent in the spring designs.
It has been common for compliant foil fluid film thrust bearings to utilize a plurality of coated, convex curved, compliant fluid foil pads that are welded to a support foil to form the fluid foil member of the bearing. These two piece fluid foil members are typically thicker and have poorer thickness control than can single piece fluid foil members. Two piece fluid foil members also experience process fluid flow turbulence, increased drag at operating speeds and reduced load capacity owing to the flow discontinuities between the trailing edges of each foil pad and the weld attachment edge of the next circumferentially located pad.
Some conventional, compliant foil fluid film thrust bearings utilize spring foil elements that are formed by milling (chemically or otherwise) circumferentially offset recesses in opposing sides of flat foil stock so as to leave circumferentially offset unmilled ridges on opposing sides of the foil elements. Pressure applied to the offset ridges induces the spring foil element to deflect in a spring-like manner. Spring foil elements formed in this manner are prone to large variations in their spring rates due to small variations in milling depth. This milling process non-symetrically relieves any residual surface compressive stresses induced by previous foil rolling operations and thus induces foil warpage.
Other bearings utilize convolute shaped spring foil elements that are formed by pressing annealed Inconel 750X foil blanks between two contoured plates having matching wavy contours with constant plate to plate spacing and then heat treating the foil blanks at approximately 1300 degrees Fahrenheit for approximately 20 hours while they are still pressed between the contoured plates. Spring foils formed in this manner are prone to have large variations in undeflected thickness.
In some cases, the fluid foils may be attached to the spring foils by welding or brazing or various spring foil elements may be welded or brazed together to form a spring foil member. Those thrust bearings that utilize welding or brazing to attach one foil element to another are subject to foil distortions and foil fatigue failures, particularly at the bond sites.
The sides of the fluid foils that face the rotating element of the bearing can utilize low rubbing friction coatings to minimize bearing wear when disk speed is below the lift-off/touch-down speed. These coatings, however, may have large thickness tolerances that can adversely affect the foil pack thickness tolerance.
The latest development in compliant foil fluid film thrust bearings, described in U.S. Pat. No. 5,529,398 issued Jun. 25, 1996 to Robert W. Bosley entitled "Compliant Foil Hydrodynamic Fluid Film Thrust Bearing" includes a self shimming capability to compensate for variations in foil pack thickness and three (3) spring or support foils beneath the fluid foil.